Custom Post Title Returns 'Array Array'

First and most important, I have no coding experience and my knowledge is limited to copy/pasting codes and make small tweaks.

Goal:
Information submitted using a CRED form to generate a custom post title.

Issue:
When using the below code in the functions.php file in my theme, it outputs "{Spotlight} Array by Array"

I need to figure out how to get it to replace the word array with the data submitted in the CRED form.

 
add_action('cred_save_data', 'build_title', 10, 2);
 
function build_title($post_id, $formdata)
{
if ($formdata['id']==8930)
{
$field1=get_post_meta($post_id, 'book-title');
$field2=get_post_meta($post_id, 'author');
 
$post_title="{Spotlight} $field1 by $field2";
wp_update_post(array('ID'=>$post_id, 'post_title'=>$post_title));
}
}

If I add ", true" as shown below, the output post title ignores these fields altogether

$field1=get_post_meta($post_id, 'book-title');
$field2=get_post_meta($post_id, 'author');

Could someone please tell me where I have gone wrong and how to resolve this?